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Old 06-11-10, 01:42 AM   #1
eventhorizon
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lost control on descent

After a few miles of riding my new bicycle that had been fitted by the LBS, I encountered a descent (not my first). I had the bike in high gear and had the brakes gently applied. The bike was accelerating on the descent and the braking wasn't helping much. Unfortunately, the front wheel started wobbling as the bike picked up speed and then I fell off the bike because of the uncontrollable wobble. Luckily, I didn't crash into anyone or anything. I sustained some scrapes and bruises, and my bicycle sustained some scrapes too. Since it has a carbon fork and seatpost, I don't plan to ride it until the LBS checks it.

How would they know whether to replace the carbon parts or not? What should we look for?

The frame is aluminum and it didn't sustain any visible damage. Derailleurs seem ok. The front brake/wheel is out of alignment. The front reflector broke off. The handlebar is no longer perpendicular to the direction of motion -- it's off by a few degrees.

Any tips on how I should handle this problem? Is this shimmy? Jobst Brandt says that adjusting the bike won't stop shimmy and others claim that it is inherent in the geometry. I have a Felt Speed 30. Was it a bad choice? Any tips on next steps would be greatly appreciated.

Thanks!
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Old 06-11-10, 04:05 AM   #2
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Wobbling.? It's either your bike handling or else some tightening needs to be done with the head tube's internal headset. A shame , so early in the life of your new bike. Good, it that it sound you are ok.?.
I've not experienced this 'wobble,' but my accident involved hitting some small pebbles at high speeds. I hit no objects when I crashed, other than the road itself. But, that was enough to cause a rip in my right shoulders' rotor cuff.
Your bike was new.? And possibly you had a problem with your bikes headset.? That is what I would suspect.
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Old 06-11-10, 04:26 AM   #3
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^Some would disagree with that latter diagnosis. Here's another perspective.
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Old 06-11-10, 04:35 AM   #4
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A fun and semi related video:

http://www.youtube.com/watch?v=6Fa0GmdSN4A
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Old 06-11-10, 04:37 AM   #5
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BeCause anyone of us has not experienced such kinds of shimmy means little.. But, sounds like his bike was pretty new.. Frame misalignment or wheels out of true..
Would you not have noticed that before a decent. ?. I certainly expect more from a new bike. Maybe the poster need contact Felt Bicycles.. ?. Anyway , he is OK and with minimal damage to his bike.. After a crash, likely some shop will say, the bike is now no longer in it's original condition , so if there was something wrong with the bike as sold; they can shift the blame to other causes. ?
. After my crash caused by hitting pebbles at high speeds, I am never the same when coming down a mountain. Small pebbles are hard to see much in advance when you are barreling down at 35 mph.
Interesting piece Metzinger.. It's lesson's as shown below . Thanks for posting. Yes. Once a year, well traveled bikes need a look over by a professional. ?>


Shimmy is not related to lo frame alignment or loose bearings, as is often claimed. Shimmy results from dynamics of front wheel rotation, mass of the handlebars, elasticity of the frame, and where the rider contacts the bicycle. Both perfectly aligned bicycles and ones with wheels out of plane to one another shimmy nearly equally well. It is as likely with properly adjusted bearings as loose ones. The idea that shimmy is caused by loose head bearings or frame misalignment seems to have established currency by repetition, although there is no evidence to link these defects with shimmy.

Bicycle shimmy is the lateral oscillation of the head tube about the road contact point of the front wheel and depends largely on frame geometry and the elasticity of the top and down tubes. It is driven by gyroscopic forces of the front wheel, making it largely speed dependent. It cannot be fixed by adjustments because it is inherent to the geometry and elasticity of the bicycle frame
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Old 06-11-10, 07:08 AM   #6
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I'd also be VERY concerned, that you said the brakes didn't seem to help, at all................

Is it possible that one, or both, wheels were not tight?.
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Old 06-11-10, 12:15 PM   #7
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I'd also be VERY concerned, that you said the brakes didn't seem to help, at all................

Is it possible that one, or both, wheels were not tight?.
The brakes were helping. I didn't want to apply too much braking because I didn't want the wheels to lock.

I am not sure what you mean by wheels were not tight. I tightened the front wheel quick release knob all the way and then engaged it. The LBS told me that if I start feeling resistance half way through engaging the quick release, then the knob is tight enough. I didn't touch the rear wheel.
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Old 06-11-10, 12:22 PM   #8
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Originally Posted by cyclezealot View Post
Wobbling.? It's either your bike handling or else some tightening needs to be done with the head tube's internal headset. A shame , so early in the life of your new bike. Good, it that it sound you are ok.?.
I've not experienced this 'wobble,' but my accident involved hitting some small pebbles at high speeds. I hit no objects when I crashed, other than the road itself. But, that was enough to cause a rip in my right shoulders' rotor cuff.
Your bike was new.? And possibly you had a problem with your bikes headset.? That is what I would suspect.
I don't believe I hit any pebbles at that time.

Could you explain the bike handling part? I would like to learn what I may have done incorrectly.

Thanks.
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Old 06-11-10, 01:49 PM   #9
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I don't believe I hit any pebbles at that time.

Could you explain the bike handling part? I would like to learn what I may have done incorrectly.

Thanks.
I didn't mean anything other than maybe you are new to cycling . Newbies to cycling might be uncomfortable to cornering if descending at fast speeds. Fast cornering is not to be taken for granted. . I might be somewhere near 100 K miles in my time cycling and fast cornering is something I am uncomfortable with and rarely do. But, then racing is not important to me.
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Old 06-11-10, 02:23 PM   #10
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"The brakes were helping. I didn't want to apply too much braking because I didn't want the wheels to lock."



Hmmm.
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Old 06-11-10, 02:59 PM   #11
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Perhaps check the tires for good mounting as one of your checks. With new rubber sidewall tires, the tire bead does not easily slide into the rim edge. This can easily destabilize the bike at higher speed.
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Old 06-12-10, 06:05 AM   #12
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Brand new brakes and rims need a bit of bedding in. You shouldn't do any safety critical braking until you have got rid of any surface gunge on the rim and pads. I usually find a steady hill and ride down with the brakes rubbing, making sure I dont go too fast.
With any new bike, you need to guage the braking capacity before you use them in anger. The best solution for brakes which are too grabby, if the front end starts to lose traction or shimmy is to release the front brakes, keep applying the rear, and try again.
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Old 06-12-10, 08:32 AM   #13
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And, holding a leg against the top tube, will help absorb some of the oscillation.

Recheck your quick releases on the wheels. Both of them.

When you tighten the quick releases, if they don't leave an imprint in your hand - they aren't tight enough!

Also remember - if you are going toooooo fast, apply more brake, while trying to keep your weight as far back as you can.
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Old 06-12-10, 08:28 PM   #14
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Originally Posted by Wanderer View Post
And, holding a leg against the top tube, will help absorb some of the oscillation.

Recheck your quick releases on the wheels. Both of them.

When you tighten the quick releases, if they don't leave an imprint in your hand - they aren't tight enough!

Also remember - if you are going toooooo fast, apply more brake, while trying to keep your weight as far back as you can.
Folks,

Thank you for your responses and support. Really appreciate it.

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With any new bike, you need to guage the braking capacity before you use them in anger.
Not sure if this is a biking expression, but I was not angry at all during the ride. Very happy, in fact, until the shimmy and fall. After that, my first thought was to get home and treat the injuries. I wasn't angry at all.

When I go to the LBS, I'll request them to check all the things all of you have outlined.

And I may have been confusing when I said wobble. By wobble, I meant that the handle bar and front wheel were quickly moving left and right like in the following video:

http://www.youtube.com/watch?v=xODNzyUbIHo

Please note that I was holding on to the grips unlike the person in the video.

Thanks!
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Old 06-12-10, 09:19 PM   #15
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Not sure if this is a biking expression, but I was not angry at all during the ride.
I think it is a reference to a military expression, to shots fired in anger (in an armed conflict) as opposed to in training. The suggestion is to test the brakes while shopping for the bike and then practice with them to become confident with them before you need to use them in a situation such as you describe. Seems to me that you got a little too close to the black hole on one of your first outings and passed through the event horizon!

How fast were you going? How steep was the descent? Were you on a road or off road? Was the surface smooth or rough? Your bike may have a problem, but maybe you just need more experience with the type of terrain you were on or the speed you were going. When you get back on the road approach similar conditions with more caution until you are confident you can handle them. And use your brakes!

Ken
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Old 06-13-10, 02:59 AM   #16
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@ eventhorizon, can you post your current measured front and rear tyre pressures?
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Old 06-13-10, 04:34 AM   #17
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I think it is a reference to a military expression, to shots fired in anger (in an armed conflict) as opposed to in training.
ok, now I understand.

Quote:
The suggestion is to test the brakes while shopping for the bike and then practice with them to become confident with them before you need to use them in a situation such as you describe. Seems to me that you got a little too close to the black hole on one of your first outings and passed through the event horizon!


Quote:
How fast were you going? How steep was the descent? Were you on a road or off road? Was the surface smooth or rough? Your bike may have a problem, but maybe you just need more experience with the type of terrain you were on or the speed you were going. When you get back on the road approach similar conditions with more caution until you are confident you can handle them. And use your brakes!

Ken
It seemed faster than the other descents that I attempted on that bike path. And that particular descent was longer than the others though not much steeper. There were no turns so I could have managed if the bicycle hadn't started shimmying. It was completely on road and smooth pavement. Here is the exact spot:

http://maps.google.com/maps?q=37.273...,0.000936&z=20

Quote:
can you post your current measured front and rear tyre pressures?
I haven't ridden the bicycle in the last week because I am recovering from my injuries and need to take the bicycle to the LBS for a checkup before riding it again.

At the time of the crash, I had just got the bicycle from the LBS. The LBS person had checked the bicycle by riding it and checking it in the store. They had adjusted the bicycle for my height, inseam, etc. The LBS person told me that I should inflate to 100psi and that he had inflated it to that level.

I had ridden the bicycle in fairly level roads for several hours before purchasing it. I would be very happy if this is an operator (i.e., my) error. If the bicycle can be fixed (it has sustained some scrapes), I can then learn how to ride if better. However, if there is some inherent problem with the geometry (Jobst Brandt says that shimmy is largely dependent on geometry and tube elasticity http://www.sheldonbrown.com/brandt/shimmy.html), then I may experience this again in the future. That makes me less confident.
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Old 06-13-10, 07:23 AM   #18
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It sounds like it might have been shimmying. However it also sounds like you can do things while riding to avoid this occurrence again. Resting a leg against the top tube will usually stop the shimmying, and it sounds like you might want to grow more comfortable with breaking on the descent. It's better to come to a full stop midway down a hill then to crash.
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